# C++ named requirements: LessThanComparable

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< cpp‎ | named req

The type must work with `<` operator and the result should have standard semantics.

### Requirements

The type `T` satisfies LessThanComparable if given expressions a, b and c of type `T` or const T(since C++11), the following expression is valid and has its specified effects:

Expression | Type | Effects
--- | --- | ---
a < b | meets BooleanTestable (until C++20) | Establishes strict weak ordering relation with the following properties: For all a, !(a < a) yields true. If a < b, then !(b < a). If a < b and b < c, then a < c. Defining equiv(a, b) as !(a < b) && !(b < a), if equiv(a, b) and equiv(b, c), then equiv(a, c).
models `boolean-testable` (since C++20)

### Notes

To satisfy this requirement, types that do not have built-in comparison operators have to provide a user-defined operator<.

For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between

- Equality, which is the value of the expression a == b and
- Equivalence, which is the value of the expression !(a < b) && !(b < a).

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2114 (P2167R3) | C++98 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened

### See also

Compare | a BinaryPredicate that establishes an ordering relation (named requirement)
strict_weak_order (C++20) | specifies that a `relation` imposes a strict weak ordering (concept)

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